There is a significant effort taking place in many parts of the world to produce transportation fuels, particularly gasoline and distillate fuels, from renewable energy sources. For example, research and development in Fischer-Tropsch technology has been on-going for decades to produce diesel fuels from syngas derived from natural gas and coal. More recently, there is a significant effort taking place to convert renewable resources, such as biomass and triglycerides to transportation fuels. Before biomass can be converted to a transportation fuel via the Fischer Tropsch or similar process (synthetic fuel, or synfuel), it must first be converted to a syngas comprised primarily of H2 and CO, which can then be sent to downstream processing to produce various chemical and transportation fuel products. Conversion of biomass to syngas is typically accomplished by gasification that converts the biomass into predominantly carbon monoxide and hydrogen (syngas) by reacting the carbonaceous material of the biomass, at high temperatures, with a controlled amount of oxygen and/or steam. The resulting syngas can be, inter alia, burned directly in internal combustion engines, used to produce methanol and hydrogen, or methanol and dimethyl ether, or converted via the Fischer-Tropsch process into synthetic transportation fuels.
Syngas produced from biomass has a different characteristic composition than syngas produced from coal or natural gas because of differences in the heating value and chemical composition of biomass compared with coal and natural gas. Specifically, syngas produced from biomass has a significantly lower H2/CO ratio than syngas produced from natural gas because biomass has a lower heating value and is deficient in hydrogen relative to that of natural gas. As such, processes developed to convert high ratio (H2/CO>2) syngas from natural gas are typically inefficient when applied to converting syngas derived from biomass. One way to overcome this problem is to “shift” the ratio of syngas produced from biomass to a higher H2/CO2 ratio via the water-gas shift reaction. This can result in higher H2/CO ratio syngas, but can be thermally inefficient because the reaction itself is exothermic and because of the requirement to produce steam, which results in lower thermal efficiency and lower carbon yield to product. There are two well established processes for converting syngas to liquid transportation fuels. One is the Fischer Tropsch process that is used to convert syngas to diesel fuel and typically utilizes syngas having an H2/CO ratio greater than 2:1. Another is the methanol to gasoline, or MTG process for producing gasoline from syngas via a methanol intermediate. Production of methanol also requires a syngas having a H2/CO ratio greater than 2. Therefore, a need exists for a an improved process that can efficiently utilize low H2/CO ratio syngas produced from biomass, or other carbonaceous feedstocks, having a relatively low heating value.